1. Field of the Invention
The present invention relates to a thin film electroluminescent (EL) device which emits a luminescence in response to the application of an electric field, and more particularly to such a device which comprises ZnS as a host material and a rare earth element providing luminescent centers.
2. Description of the Prior Art
The thin film EL devices presently in use comprise an EL film which is composed of ZnS serving as a host material and doped with Mn providing luminescent centers. These devices, however, are limited to yellowish organge in the color of luminescence. Accordingly, EL devices are desired which luminesce in the three primary colors, i.e. red, green and blue, as required for realizing a full-color luminescence. For this purpose, research is conducted on the use of rare earth elements as luminescent centers. For example, Tb, Sm and Tm, when used, are thought to produce green, red and blue luminescences, respectively.
Such an EL film comprising the host material ZnS doped with a rare earth element is prepared usually by radio-frequency (rf) sputtering or electron beam vacuum evaporation using these materials, i.e. ZnS and a halide or oxide of the rare earth element, in combination. For example, an EL film (ZnS: Tb, F) prepared from a target consisting of the mixture of ZnS and the fluoride of a rare earth element (e.g. TbF.sub.3) by a sputter technique is known to have some degree of luminescence brightness (Unexamined Japanese Patent Publication SHO No. 61-273894).
However, conventional EL devices comprising such an EL film containing the rare earth element still remain to be improved in luminescence brightness and efficiency for actual use.
For example, with reference to the broken line in FIG. 1 representing the optical excitation spectrum of the above-mentioned ZnS:Tb, F EL film (Tb concentration: about 2 at. %) which produces a green luminescence, the spectrum has excitation bands at about 380 nm and about 335 nm, but the intensity of excitation is low.
Although the excitation mechanism at the luminescent centers has yet to be fully explained, it appears likely that hot electrons accelerated by a high electric field and having high energy will collide with the luminescent centers (direct collision excitation), or the recombination energy of electron-hole pairs released from the host ZnS upon impact ionization will be delivered to the luminescent centers by resonance transfer (band excitation). Accordingly, it is thought that the above-mentioned excitation bands correspond to these respective excitation mechanisms. Nevertheless, in the case of the direct collision excitation of luminescent centers of the rare earth element, it is difficult to achieve a greatly improved excitation efficiency since the luminescence of the rare earth ion is inherently due to forbidden transition. Further in the case of band excitation, the recombination energy of electron-hole pairs is transferred to the rare earth ion only with a very low efficiency and is predominantly converted to thermal energy. Presumably for these reasons, the conventional EL film is unable to exhibit a high excitation intensity at either of the excitation bands as indicated in the broken line in FIG. 1, failing to give sufficient luminescence brightness.
To obviate the drawback, it is required to improve the crystallinity of the host by resorting to film forming techniques other than the conventional sputtering process and vacuum evaporation process, whereas difficulties are encountered in fulfilling this requirement in respect of amenability to quantity production and cost.
The main object of the present invention, which has been accomplished in view of the above problem, is to improve the luminescence brightness of EL films having luminescent centers afforded by a rare earth element.